As illustrated in FIG. 11, a conventional X-ray diagnostic apparatus includes an X-ray tube 103 that irradiates a subject M with X-rays; an X-ray detector (e.g., a flat panel X-ray detector (FPD)) 104 that detects X-rays transmitting through the subject M; and a scattered-radiation removal grid (hereinunder, abbreviated as “grid”) 105 disposed on a side where X-rays enter into the X-ray detector 104. The grid 105 removes X-rays scattered upon transmitting through the subject M. The grid 105 is formed by absorbers (e.g., lead) and transmission elements (e.g., aluminum or air) arranged alternately, the absorbers absorbing X-rays and the transmission elements transmitting X-rays. In the grid 105, the absorbers absorb X-rays scattered and entering obliquely, and the X-ray detector 104 only detects X-rays transmitting through the transmission elements. This enables to obtain a clear image.
In the conventional X-ray diagnostic apparatus 101, however, a moiré pattern of the grid 105 appears in the obtained image due to difference between resolution of the X-ray detector 104 and density of the grid 105. Such a problem may arise. Various suggestions have been made to take measures against the moiré pattern of the grid 105. See, for example, Japanese Patent Publication No. 2002-152467A.
Moreover, the conventional X-ray diagnostic apparatus 101 as illustrated in FIG. 11 includes a moiré pattern remover 121. The moiré pattern remover 121 removes a first harmonic and a second harmonic in the moiré pattern of the grid 105 appearing in the image. First, one-dimensional Fourier transform is performed to the image with the moiré pattern appearing therein. Then, in accordance with results from the Fourier transform, a peak frequency representing the first harmonic in the moiré pattern is detected. Here, the second harmonic is twice (integral multiple) the first harmonic, and thus enables to be calculated from the peak frequency of the first harmonic. Then, in accordance with the peak frequencies of the obtained first and second harmonics, a filter is prepared to extract the first and second harmonics in the moiré pattern. The first and second harmonics are extracted using the filter. Thereafter, the first and second harmonics are subtracted from the image with the moiré pattern appearing therein. As a result, the first and second harmonics in the moiré pattern are removed.
The resolution of the X-ray detector 104 enables to be expressed by a Nyquist frequency Ny. Here, a Nyquist frequency Ny represents a limit of spatial frequencies that enables to be sampled at pixel pitches δ (mm). The Nyquist frequency Ny is calculated from Equation (1) as under:Ny=1/(2×δ)  (1)For instance, assuming that the X-ray detector 104 has pixel pitches δ of 150 μm, a Nyquist frequency Ny is calculated:Ny=1/(2×0.15)=3.33 . . . lp/mm.That is, the resolution of the X-ray detector 104 is 3.33 . . . lp/mm. Here, the resolution of the X-ray detector 104 is appropriately assumed to be “3.33 lp/mm”.Moreover, the density of the grid 105 is, for example, 50 line/cm. Here, 50 line/cm is also expressed by 5 line/mm or 5 lp/mm. As for the grid 105, “line/cm” or “lp/mm” represents the number of absorbers or the number of pairs of the absorber and transmission element (e.g., 50 or 5) per unit of length (e.g., 1 cm or 1 mm). When the X-ray detector with the resolution of 3.33 lp/mm and the grid with the density of 5 lp/mm (50 line/cm) are used, a moiré pattern of the grid (5 lp/mm) is folded at 3.33 lp/mm and thus occurs in a position of 1.67 lp/mm. In other words, a first harmonic in the moiré pattern of the grid appears in a position of 3.33 . . . lp/mm−(5 lp/mm−3.33 . . . lp/mm)=1.67 (1.666 . . . ) lp/mm.